3 or 4 pole ATS?

[mparkstexas]

I am working on an existing 600A MCB, 277/480V, 3 phase, 4 wire distribution panelfed from a central plant. We are taking it off the plant and placing it on a utility transformer. We are also adding a 125 kVA standby generator.

I am providing a service entrance rated ATS, but I am having trouble understanding the pros/cons of a 3 pole vs 4 pole ATS in this situation. As I understand it, the whole point of a 4 pole is to switch the neutral to help with the GFI protection in services over 1000A.

Since mine is under 1000A, I am planning on going with a 3-pole. Is my understanding incorrect? Does this leave potential for neutral currents in de-energized equipment?

Appreciate the help in advance.

 

[charlie b]

A key difference between the 3-pole and the 4-pole relates to the connections of the equipment grounding conductors. Using the 4-pole allows you to make the backup generator a "separately derived system. I prefer the simplicity of that arrangement. I think the Handbook has a couple figures that show the different ways to connect ground-related wires in the two configurations, but I haven't the time right now to look for them.

 

[mparkstexas]

I have been looking at 250.6, but my problem arises in the fact that I am using the ATS as the service equipment. It seems to me that since you are already running the neutral and the equipment ground from the generator to the ATS, you might as well make it a 3 pole and save yourself the money and extra grounding electrode required for the 4 pole ATS.

Any thoughts?

 

[skeshesh]

There's some trade-off of advantages/disadvantages between 3 & 4 poles xfer switches. I think the main issue is that the 3-pole ATS has issues with isolating faults that could propogate thru the neutral; the 4-pole ATS could create switching issues during the transition, and this could both irregular voltage conditions as well as effect equipment that use rectifiers/other power conversions that are sensitive to input voltage turbulences, such as UPSs, etc. In some applications there really isn't a major disadvantage. For a quick overview, EC&M has a short article summarizing some of the points:

http://ecmweb.com/mag/electric_switching_neutral_whats/

 

[mparkstexas]

I dont have any sensitive equipment, no UPS, servers, etc... would you still stick with a 3 pole?

I'm trying to figure out if the neutral can carry current through a 3 pole, how is it ever code compliant?

 

[skeshesh]

I dont have any sensitive equipment, no UPS, servers, etc... would you still stick with a 3 pole?

I'm trying to figure out if the neutral can carry current through a 3 pole, how is it ever code compliant?

I don't really know what you mean by this question. The neutral (grounded conductor) sometimes carries load unbalance but is generally not a current carrying conductor.
You could use a 3-pole if you're not really concerned with the issues with the neutral - it's really a design issue rather than a code issue. Also check local codes. Here in LA the city code requires using a 4-pole ATS when transfering to an emergency source.

 

[LarryFine]

I dont have any sensitive equipment, no UPS, servers, etc... would you still stick with a 3 pole?

It would be my first choice.

I'm trying to figure out if the neutral can carry current through a 3 pole, how is it ever code compliant?

Because, with the line conductors deenergized, no current will flow on that section of the neutral.

It's the same way your service disconnect at home doesn't need to open the neutral.

 

[mparkstexas]

Perfect, thanks for the responses!

 

[dbuckley]

Late response: where possible, I'm a big fan of three pole, and then you can bolt all the neutral and ground stuff together solid as a rock. This gives the neutral path a slightly lower impedence then you get in a switched neutral arrangement.

I have never seen any advantage of a four pole system when a three will do when you're talking a mechanical transfer switch. When you have static (semiconductor) switches in a multiple source environment then suddenly I'm a defector and in the four pole switching camp, as that can protect you against overvoltage caused by an upstream lost neutral.

Obviously if you are feeding multiple services from a common genset or some other oddball (but none-the-less common) scernario then four pole switching is required and you just gotta go with it.

 

[benaround]

Late response: where possible, I'm a big fan of three pole, and then you can bolt all the neutral and ground stuff together solid as a rock. QUOTE]

Except at the generator, the neutral and the egc must stay seperated, and no ground rod.

Only other thing is, on gen. power, a fault to ground from the load has to go to the service

panel 1st then jump on the neutral and go back to the gen.

 

[SG-1]

Do not forget about 700.8(B) or 701.9(B) or 702.8(B) one of which applies.

 

[jghrist]

Except at the generator, the neutral and the egc must stay seperated, and no ground rod.

Only other thing is, on gen. power, a fault to ground from the load has to go to the service

panel 1st then jump on the neutral and go back to the gen.

If the ATS is the service panel, this is not an issue.